Ionothermal confined self-organization for hierarchical porous magnesium borate superstructures as highly efficient adsorbents for dye removal

Abstract

Three-dimensional (3D) hierarchical porous structures self-organized by one-dimensional (1D) building blocks have attracted considerable attention due to their unique properties and versatile applications. Herein, uniform hierarchical porous MgBO₂(OH) superstructures with a microsphere-like profile were synthesized for the first time via a facile ionothermal synthesis (150 °C, 12.0 h) using MgCl₂, H₃BO₃ and NaOH as the raw materials, and N,N-dimethylformamide nitrate ionic liquid as the solvent. The porous MgBO₂(OH) microspheres exhibited a specific surface area of 57.22 m² g⁻¹, 82.4% of which had a diameter within the range of 4.0–6.0 μm. The subsequent mild thermal conversion (600 °C, 12.0 h) gave rise to the hierarchical porous Mg₂B₂O₅ microspheres with high crystallinity and a specific surface area of 24.20 m² g⁻¹. The synergistic effect of the hydrophilic surfaces of the MgBO₂(OH) nanostructures and the ionothermally confined self-organization co-contributed to the formation of the hierarchical porous MgBO₂(OH) and Mg₂B₂O₅ superstructures, which were employed as adsorbents for Congo red (CR) in simulated waste water. Both MgBO₂(OH) and Mg₂B₂O₅ superstructures exhibited good recyclability and reusability as high efficient adsorbents for dye removal after regeneration. The Langmuir isothermal model well interpreted the adsorption behavior of CR. The intra-particle diffusion model was also well fitted based on the kinetic data, indicating microsphere diffusion as the rate-limiting step. The maximum adsorption capacities, q_m, for CR onto the porous MgBO₂(OH) and Mg₂B₂O₅ superstructures, which were calculated from the Langmuir isothermal model, were 228.3 and 139.3 mg g⁻¹, respectively. The present ionothermally confined self-organization for the hierarchical porous MgBO₂(OH) and Mg₂B₂O₅ superstructures provide new insights into the material chemistry, as well as a controllable ionothermal route for hierarchical borate nanoarchitectures. In addition, these superstructures provide a new platform for dye adsorption, and other potential fields, such as heterogeneous catalysis.